Cadmium sulfide (CdS) is a well known semiconductor with a band gap in the visible region of the spectrum. Films consisting essentially of cadmium sulfide are of use as photoconductors, piezoelectric transducers, and laser materials; and cadmium sulfide has been considered for use in nonlinear optical processing. See, e.g., an article by H. S. Kwok et al. entitled "Growth of highly oriented CdS thin films by laser evaporation deposition," appearing in Applied Physics Letters, Volume 52, No. 13, 28 March 1988.
In the Kwok et al. article, a process is disclosed in which epitaxial growth of a cadmium sulfide film upon a substrate was caused to occur by bombarding a cadmium sulfide target an argon fluoride laser. The process was conducted in a vacuum chamber with a base pressure of 10.sup.-5 Torr. An argon fluoride laser with a wavelength of 193 nanometers operating at 10 pulses per second was focused onto the cadmium sulfide target at a 45 degree angle of incidence, and the cadmium sulfide target was rotated slowly. The substrate was heated to a temperature of either 350 or 400 degrees Centigrade, and it was located about 3 centimeters above the target.
The process of the Kwok et al. article was successful in producing a structure coated with a hexagonal (greenockite) cadmium sulfide film. However, when an attempt was made to produce a structure coated with a cubic (hawleyite, zinc blende) cadmium sulfide film, such attempt was unsuccessful; see, e.g., page 1096 of the article.
There is evidence that the energy band gap for cubic cadmium sulfide is lower than the energy band gap for hexagonal cadmium sulfide. Thus, e.g., in an article by M. Huang et al., appearing in the Journal of Physical Chemistry Solids (Volume 46, No. 8, pages 977-995, 1985), it is disclosed that the energy band gap for the cubic cadmium sulfide is 2.51 electron volts, whereas the energy band gap for the hexagonal cadmium sulfide is 2.56 volts. Reference also may be had to an article by E. L. Lind et al. entitled "Photoconductivity in Cubic Cadmium Sulfide," appearing in the Journal of Chemical Physics, Volume 37, pages 2449-2450 (1962).
As is known to those skilled in the art, the energy band gap is the amount of energy required to cause an electron in a semiconductive material to jump from the valence band to the conductive band; see, e.g., pages 1112-1120 of R. G. Lerner et al.'s "Encyclopedia of Physics," Second Edition (VCH Publishers, Inc., New York, 1991). Thus, for example, substantially less energy is required to activate a cubic cadmium sulfide material than a hexagonal cadmium sulfide material; and this property is advantageous for optoelectronic devices.
It is an object of this invention to provide an economical process for producing a film containing a major amount of cubic cadmium sulfide.
It is yet another object of this invention to provide a process for producing a cubic cadmium sulfide film with excellent optical quality and homogeneity.
It is another object of this invention to provide a process for preparing a single crystal cubic cadmium sulfide thin film material.
It is yet another object of this invention to provide an optical device comprised of a single crystal cubic cadmium sulfide thin film material.